Case Study – The Variability of Effects During Rain Events. A TMDL Study to Link Metals and Diazinon to Toxicity Impacts in a Highly Urbanized Creek

The City of San Diego, the Port of San Diego, the San Diego Regional Water Quality Control Board, and the California Department of Pesticide Regulation funded a study of organophosphate pesticides and metals in the Chollas Creek watershed from 1999 to 2001. The objectives of this study were:To characterize the contaminants within the reaches of the watershed for use in TMDL development.Evaluate the relationship between toxicity observations and chemical measurements at each of the sampling stations.Identify if any region or reach within the Chollas Creek watershed is a source of contamination.The supporting agencies and stakeholders had an anticipation that this study would result in several outcomes. First it was believed that specific areas/reaches of the watershed would be found to contribute higher levels of contamination than other areas. Second, it was anticipated that the study would result in a definitive linkage between toxicity to bioassay organisms and constituents of concern. The findings of this study are being used by the San Diego Regional Water Quality Control Board in the TMDL's for Chollas Creek.The first four storm events monitored from 2000-2001 resulted in no definitive linkage between diazinon and toxicity. The regression analysis for Ceriodaphnia dubia toxicity and diazinon resulted in an r2 = 0.4896. This was the strongest relationship. There were no observed relationships between dissolved metals and toxicity to either Ceriodaphnia dubia or Hyalella azteca. Further, there was no specific area or watershed reach that definitively demonstrated greater contaminant contribution during storm events. The Department of Pesticide Regulation was very interested in capturing a first flush storm event in this study. After the 2000-2001 storm season, additional funding was made available to capture the first flush of the season and repeat the analysis of results. This first flush as captured in November of 2001 after more than six months of no measurable runoff. Only after analyzing the data resulting from the first flush event was a correlation between diazinon and bioassay organism toxicity evident. Still, no specific area or watershed reach demonstrated greater concentrations of contaminants.